golang读取切分存储byte流文件

package main
import (
    "fmt"
    "os"
    "time"
)
func check(e error) {
    if e != nil {
        panic(e)
    }
}
func cat(f *os.File) []byte {
    var payload []byte
    for {
        buf := make([]byte, 1024)
        switch nr, err := f.Read(buf[:]); true {
        case nr < 0:
            fmt.Fprintf(os.Stderr, "cat: error reading: %s\n", err.Error())
            os.Exit(1)
        case nr == 0: // EOF
            return payload
        case nr > 0:
            payload = append(payload, buf...)
        }
    }
}
func main() {
    file, err := os.Open("test.flv")
    if err != nil {
        panic(err)
    }
    defer file.Close()
    fmt.Println(file)
    payload := cat(file)
    fo, errs := os.Create(fmt.Sprintf("./%d.bmp", time.Now().UnixNano())) //time.Now().UnixNano()
    check(errs)
    _, errs = fo.Write(payload)
}